renderer: refactor the way ICC profile handling works

I'm not very happy with this, because now we have two fields living alongside each other for no real reason except me being unable to deprecate the old field as long as the libav wrappers still need to be stateless.

Being able to tie the lifetime of the ICC object directly to the pl_frame would solve this, but that's a more major undertaking than what I'm comfortable doing in v6. Maybe we can revisit this later.